Antibiotic residues (bacterial inhibitory substances) in the milk of cows treated under label and extra-label conditions

The objectives of this study were to describe the depletion pattern of antibiotic residues (microbial inhibitory substances) from the milk of cows treated under field conditions of clinical disease and antibiotic administration, including both label and extra-label use, and to determine if the type of extra-label use, the route of administration, and the drug used were factors associated with prolonged shedding of residues in milk.

Milk samples from 138 cows, treated with a variety of antibiotic products on farms in southwestern Ontario in 1989 and 1990, were collected before treatment and for six days after cessation of treatment. Samples were tested for antibiotic residues with the Brilliant Black reduction test (BR-test) and the Bacillus stearothermophilus var calidolactis disc assay. In 13/138 (9.4%) of cow treatments, at least one milk sample was positive on both antibiotic residue tests after the label milk withholding time for the drug(s) used. In ten of these instances, the antibiotics were administered in extra-label fashion and in three the drugs were reportedly used according to label instructions. Extra-label use of antibiotics was significantly associated with increased risk of antibiotic residues in milk beyond the label withholding time. No significant differences in risk were observed among the various antibiotic products used in the study.

The farms involved in this study were selected on the basis of their proximity to our laboratory; therefore, the frequency of antibiotic residue detection after withholding times may not be indicative of the provincial or national situation. If farmers and veterinarians find themselves in a situation where extra-label use of an antibiotic is necessary, use of an alternative antibiotic that can be used at label dose and with a known withdrawal time may avoid a problem with residues.